1. Field of the Invention
The present invention relates to polyolefinic fibers having improved thermal bonding properties and to the relevant process for obtaining same.
More particularly the present invention relates to polyolefinic fibers having improved thermal bonding properties, specially suitable for producing fabrics cohesioned by thermal bonding and products intended to be used for thermo-forming processes.
The term polyolefinic fibers, as used in the present disclosure and in the claims, comprises as well the fibrous materials, generally known as fibrils, fibrids and plexo filamentary fibrids, even the ones having a surface area larger than 1 m.sup.2 /g.
2. The Prior Art
As known, polyolefinic fibers are widely employed for the manufacture of products cohesioned by thermal bonding, in particular by the calendering technology.
It is also known, how the process conditions for preparing the polyolefinic fibers, affect the thermal bonding properties of the obtained fibers.
In fact it is known that the thermal bonding properties of the polyolefinic fibers improve by lowering the draw ratio and attaining the highest value in the undrawn fibers.
The production of undrawn or of partially drawn fibers, of course, reduces the productivity of the spinning machine and, therefore, the less the draw ratio the more decisive is the influence on the cost of the product.
It has been proposed to employ, for the production of thermal bonded products, bicomponent polypropylene-polyethylene fibers of "side-by-side" type, so that the polyethylene, having a lower melting point, allows an easier bonding among the fibers. The use of these bicomponent fibers, however, did not succeed to obtain satisfactory cohesioning values, because the bond among the fibers only occurs between polyethylene and polyethylene.
It was also proposed to employ fibers consisting of ethylene-propylene copolymers. These fibers succeeded to obtain satisfactory thermal bonding, by working at less critical temperatures and process conditions, with respect to the ones used for the drawn polypropylenic fibers, but the cost of these fibers is higher than the one of the usual polypropylenic fibers.